This invention relates to a protection system for non-linear varistors, and in particular it relates to a protection system for zinc oxide varistors used to prevent overvoltage on AC busses or lines.
Zinc oxide varistors are frequently used to prevent overvoltages on AC busses or on apparatus associated with such AC busses, such as for example with capacitors. Capacitors are often connected in an AC system to control phase or to provide voltage support, and the capacitors are vulnerable to overvoltages caused by faults. Such capacitors may be connected in banks comprising series and parallel connected capacitors to provide a desired total capacity and voltage requirement, and it will be understood that reference to a capacitor may include such an arrangement. Because capacitors and other equipment may be damaged by overvoltage, it is known to provide protection by connecting zinc oxide varistors across the capacitors or across other equipment to be protected. As an example of other equipment protected by varistors, reference is made to Canadian Patent No. 1,162,977--Chadwick, issued Feb. 28, 1984 to Canadian General Electric Company Limited which describes apparatus having thyristors protected by zinc oxide varistors.
When zinc oxide varistors are used to protect capacitors or other equipment having high voltages applied, the varistors may be arranged with a number in series, and may be surrounded by an insulator. This assembly can be referred to as a column. The number of varistors in series is selected to achieve a required design level of voltage, and there may be two or more columns in parallel.
A varistor is selected so that there is only a very small current flow at normal load, and as the voltage increases above normal there is an increased current flow which tends to limit the voltage. Columns of varistors function in the same manner as individual varistors and thus provide protection against overvoltages on busses to which capacitors or other devices are connected. However, the zinc oxide varistor has a limit to the energy it can handle before the varistor is damaged, and the limit is related to temperature and to incremental temperature. It is desirable to shut down the equipment by, for example, tripping a breaker, or alternatively to limit the varistor current in some manner before the varistor is damaged.
Canadian Patent No. 1,123,895--Hamann, issued May 18, 1982 to General Electric Company, describes a protection system for varistors where the current flowing through the varistors is monitored. A thermal analog circuit receives a signal representing the monitored current and determines if the monitored current represents an amount of energy being dissipated in the varistor which exceeds a predetermined value. If the calculated dissipation exceeds this predetermined value, the thermal analog circuit provides a trigger signal to an air gap device connected across the varistor causing the air gap to conduct. The thermal analog circuit may also determine when the monitored current represents a rate of rise of energy dissipation that is excessive, that is the determined rate of rise of current exceeds a predetermined level, and if so it provides a triggering signal to cause the air gap to conduct. When the air gap conducts, it rapidly lowers the voltage across the varistor and hence limits the rate of rise of the current.
While the system of Hamann provides protection, the protection is based only on current and the temperature must be determined from the current and from the changing ambient and operating conditions. In other words, the temperature is determined indirectly. More accurate protection is attainable by measuring actual temperature and by determining the total energy involved rather than rate of dissipation.